1. Field of the Invention
This invention relates generally to the field of loudspeakers. In particular, the invention relates to a variable impedance multiple voice-coil electromagnetic loudspeaker.
2. Related Art
A loudspeaker typically includes a frame, a motor system that provides a magnetic field across an air gap, a voice-coil, a former for supporting the voice-coil in the air gap, a diaphragm having an outer perimeter and an apex, and a surround coupled to the outer perimeter and the frame to support the outer perimeter from the frame. The voice-coil, supported by the former, is coupled to the apex of the diaphragm so that the current that flows through the voice-coil and causes the voice-coil to move in the air gap also causes the diaphragm to move.
The motor system functions as an electro acoustic transducer (also referred to as simply a transducer or loudspeaker). The motor system typically includes a magnet (typically a permanent magnet) and associated ferromagnetic components—such as pole pieces, plates, rings, and the like—arranged with cylindrical or annular symmetry about a central axis. However, any device that creates a static magnetic field may be used, including field coil motors with no permanent magnets. Moreover, other arrangements may be used, such as square, race track or other asymmetric configurations.
Taking the annular configuration as a typical example, the motor system establishes a magnetic circuit in which most of the magnetic flux is directed into an annular (circular or ring-shaped) air gap (“magnetic gap” or “magnetic field”), with the lines of magnetic flux having a significant radial component relative to the axis of symmetry. The voice coil typically is formed by an electrically conductive wire cylindrically wound for a number of turns around a coil former. The coil former and the attached voice coil are inserted into the magnetic gap of the motor system such that the voice coil is exposed to the static (fixed-polarity) magnetic field established by the motor system. The voice coil may be connected to an audio amplifier or other source of electrical signals that are to be converted into sound waves. A rigid loudspeaker diaphragm (often called a cone due to the typical shape) is suspended by one or more supporting but compliant elements of the loudspeaker, e.g., a surround, spider, or the like, such that the flexible portion permits the rigid diaphragm to move. The diaphragm is mechanically referenced to the voice coil, typically by being connected directly to the coil former on which the voice coil is supported.
In operation, electrical signals are transmitted as an alternating current (AC) through the voice coil in a direction substantially perpendicular to the direction of the lines of magnetic flux produced by the magnet. The alternating current produces a dynamic magnetic field, the polarity of which flips in accordance with the alternating waveform of the signals fed through the voice coil. Due to the Lorenz force acting on the coil material positioned in the permanent magnetic field, the alternating current corresponding to electrical signals conveying audio signals actuates the voice coil to reciprocate back and forth in the air gap and, correspondingly, move the diaphragm to which the coil (or coil former) is attached. Accordingly, the reciprocating voice coil actuates the diaphragm to likewise reciprocate and, consequently, produce acoustic signals that propagate as sound waves through a suitable fluid medium such as air. Pressure differences in the fluid medium associated with these waves are interpreted by a listener as sound. The sound waves may be characterized by their instantaneous spectrum and level, and are a function of the characteristics of the electrical signals supplied to the voice coil.
A loudspeaker transducer is associated with a nominal impedance. The electro-acoustic characteristics of the transducer depend on its net or nominal impedance. In some settings, it may be desirable to vary the nominal impedance of the loudspeaker. Currently commercially available dual voice coil (“DVC”) loudspeakers may be configured to allow a user to switch between one of two different loads: (1) between a 2-ohm and 8-ohm load or (2) between a 1-ohm and 4-ohm load. However, with these currently available switchable dual voice coil loudspeakers, it is generally only practical to use one of the two available impedance values. With a switchable 2-ohm/8-ohmloudspeaker, 8 ohms is rarely used due to its low output sensitivity. With a switchable 1-ohm/4-ohmloudspeaker, the 1-ohm load is also rarely used because it creates current delivery problems for many amplifiers. Moreover, the respective loudspeaker configurations will not have the same electrical parameters, such as electromotive force (BL2/Rc).
Alternatively, a loudspeaker system may be installed such that the loudspeakers themselves are configured either in parallel or in series, depending on the desired load. However, installation of such a system according to the required specifications for a particular setup is generally complex and can result in mistakes or complications. Thus, it is desirable to provide a simplified loudspeaker installation procedure that will allow for flexibility in impedances, will result in fewer mistakes and miscalculations in installation, and will operate under the same electrical parameters regardless of the impedance value of the load.